1. Field of the Invention
The present invention relates to a diffraction optical element and a light beam detection device, an optical scanner, and an image forming apparatus. The present invention can be applied in a digital copying machine, a printer, a facsimile device, a plotter, a digital complex machine such as a device including a hybrid function of a copying function, a facsimile function, a printer function, a plotter function, a scanning function, and the like, and a measuring device.
2. Description of the Related Art
The diffraction optical element is known as an optical element to separate a light beam by diffraction phenomenon and used in various optical devices. In general, to separate the light beam, it is necessary that the separated beams have a large separation angle therebetween. As is generally known, in a periodic structure having a diffractive surface, since a diffraction angle becomes large when a period of the periodic structure becomes small, the light beam can be separated with the large separation angle by adjusting the period of the periodic structure. However, when the period of the periodic structure is too small, deflection dependency due to structure birefringence occurs.
That is, in the diffraction optical element having the periodic structure with the period of about a wavelength of the light beam to be separated by the diffraction, the large separation angle can be achieved while the deflection dependency due to the structure birefringence occurs.
The light beam emitted into the diffraction optical element is generally in various deflected states, and if there is the deflection dependency due to the structure birefringence, the deflected state of the incident light beam generally differs from that of the separated beam. Even when the incident light beam is in a linear deflected state, except when a deflected direction is parallel or perpendicular to a period direction, the separated beam is generally in an elliptically deflected state.
Furthermore, in the case where the incident light is in an elliptically deflected state, each of the separated beams is in an elliptically deflected state different from the deflected state of the incident light beam.
The diffraction optical element is preferably a diffraction optical element which can separate the light beam regardless of the deflected state of the incident light beam, for example, can separate the light independently from the deflected state of the incident light beam.
For example, the diffraction optical element can be used in an optical scanner in which a light beam emitted from a semiconductor laser is deflected by a light deflection unit or a light deflector and scans a surface to be scanned to perform an optical writing, and can be used to detect the light beam to adjust a scanning position. In this case, in the optical scanner in which the light beam emitted from the semiconductor laser is not linearly deflected parallel or perpendicular to a deflected scanned plane which is a virtual plane where the light beam deflected by the light deflector is swept or in the optical scanner which has a reflection mirror reflecting the light beam to an outside of the deflected scanned plane, the deflected state of the light beam is gradually varied by the reflection and the light beam to be detected is generally in an elliptically deflected state.
In addition, even when an optical scanner is configured such that the light beam to be deflected is maintained in a linear deflected state in a light path, it is possible that the light beam becomes in an elliptically deflected state due to variations occurring in production of the semiconductor laser, errors occurring in mounting the semiconductor laser on the optical scanner, or errors in installing the light deflection unit or optical elements. Furthermore, if errors in attaching the reflection mirror or the optical element to which the elliptically-deflected light beam is emitted occur, the elliptically deflected state is varied in accordance with the errors.
Therefore, it is required that the diffraction optical element to which the light beam is emitted can be adjusted and used to separate the incident light beam in any deflected states.
In addition, such a diffraction optical element can be used regardless of the deflected state of the light beam so that the diffraction optical element can be commonly-used in the various optical devices and the low cost optical devices can be obtained.
Various detection methods to detect the light beam used to scan the surface to be scanned in the optical scanner are disclosed in, for example, Japanese Patent Application Publication Numbers 2005-37575, 2005-62597 and Japanese Patent Number 3191232.